A distance measurement device, or ranging device, is based on the principle that an electromagnetic pulse is transmitted towards a surface or an object, and a reflected pulse is received. The distance can then be determined based on the time-of-flight of the pulse to the surface or object and back. This determination can be performed using suitable signal processing. Ideally, the received signal would simply be sampled in order to detect an echo peak. However, as electromagnetic waves propagate with the speed of light, the echo peak will typically have a duration in the order of only a few nanoseconds. Conventional samplers are not fast enough to capture such a fast pulse with sufficient resolution to detect a distance with high accuracy.
Solutions to this problem are known, for example in the level gauging field. In level gauges employing electromagnetic pulses (here referred to as radar level gauges, RLG, even though the frequencies employed are not necessarily in the conventional radar range) the gauge transmits a pulse train comprising nanosecond pulses with a repetition frequency in the Megahertz range. The reflected pulse train is mixed it with the transmitted signal, in order to provide a time expanded pulse response. While providing a satisfactory result, this process requires a complex and relatively expensive design. Efforts have therefore been made to provide real time sampling on a time scale short enough to allow sampling of one single pulse or as few pulses as possible to map the range of the signal (e.g. 32 pulses for 5-bit resolution).
As one alternative, it has been proposed to use a radar module integrated on one silicon chip. Such a module can allow sampling in real time of a pulse with a time duration in the order of ns or less, with a known time relationship between transmitted signal and received signal.
According to one operating principle for such a single chip radar module, a received pulse is compared to a threshold level and sampled by a large number (e.g. 128) successive samplers beginning at a given point in time (strobed sampling) and ending at another point in time, such that the sampling covers a time window typically longer than the pulse itself. By repeating the sampling several times, while gradually increasing (sweeping) the threshold, the amplitude of the received signal can be recovered. This is referred to as “swept threshold sampling” and is described in the article “Thresholded samplers for UWB impulse radar” by Hjortland et al. An example of a radar chip based on this principle is commercially available from Novelda in Norway.
A potential problem with sampling a signal in real time using a series of on-chip elements, is the absence of a temperature-stable clock reference. Although the starting point of sampling can be established e.g. with a reference echo, the exact duration of the sampling will depend on the delay components on the chip. For example, the radar module may exhibit a large temperature drift, since all delay elements in the chip possess pronounced temperature dependence.
In laboratory measurements an uncompensated temperature drift in the order of 4 cm per 10° C. has been observed. This problem is particularly relevant for radar level gauging implementations.